Showing papers on "Magnetic flux published in 2014"

Non-reciprocal phase shift induced by an effective magnetic flux for light

Abstract: Photons are neutral particles that do not interact directly with a magnetic field. However, recent theoretical work has shown that an effective magnetic field for photons can exist if the phase of light changes with its direction of propagation. This direction-dependent phase indicates the presence of an effective magnetic field, as shown experimentally for electrons in the Aharonov–Bohm experiment. Here, we replicate this experiment using photons. To create this effective magnetic field we construct an on-chip silicon-based Ramsey-type interferometer. This interferometer has been traditionally used to probe the phase of atomic states and here we apply it to probe the phase of photonic states. We experimentally observe an effective magnetic flux between 0 and 2π corresponding to a non-reciprocal 2π phase shift with an interferometer length of 8.35 mm and an interference-fringe extinction ratio of 2.4 dB. This non-reciprocal phase is comparable to those of common monolithically integrated magneto-optical materials.

Flux Emergence (Theory)

Abstract: Magnetic flux emergence from the solar convection zone into the overlying atmosphere is the driver of a diverse range of phenomena associated with solar activity. In this article, we introduce theoretical concepts central to the study of flux emergence and discuss how the inclusion of different physical effects (e.g., magnetic buoyancy, magnetoconvection, reconnection, magnetic twist, interaction with ambient field) in models impact the evolution of the emerging field and plasma.

Hall effect controlled gas dynamics in protoplanetary disks. ii. full 3d simulations toward the outer disk

Abstract: We perform three-dimensional stratified shearing-box magnetohydrodynamic (MHD) simulations on the gas dynamics of protoplanetary disks with a net vertical magnetic flux of B z0. All three nonideal MHD effects, Ohmic resistivity, the Hall effect, and ambipolar diffusion, are included in a self-consistent manner based on equilibrium chemistry. We focus on regions toward outer disk radii, from 5 to 60?AU, where Ohmic resistivity tends to become negligible, ambipolar diffusion dominates over an extended region across the disk height, and the Hall effect largely controls the dynamics near the disk midplane. We find that at around R = 5?AU the system launches a laminar or weakly turbulent magnetocentrifugal wind when the net vertical field B z0 is not too weak. Moreover, the wind is able to achieve and maintain a configuration with reflection symmetry at the disk midplane. The case with anti-aligned field polarity () is more susceptible to the magnetorotational instability (MRI) when B z0 decreases, leading to an outflow oscillating in radial directions and very inefficient angular momentum transport. At the outer disk around and beyond R = 30?AU, the system shows vigorous MRI turbulence in the surface layer due to far-UV ionization, which efficiently drives disk accretion. The Hall effect affects the stability of the midplane region to the MRI, leading to strong/weak Maxwell stress for aligned/anti-aligned field polarities. Nevertheless, the midplane region is only very weakly turbulent in both cases. Overall, the basic picture is analogous to the conventional layered accretion scenario applied to the outer disk. In addition, we find that the vertical magnetic flux is strongly concentrated into thin, azimuthally extended shells in most of our simulations beyond 15?AU, leading to enhanced radial density variations know as zonal flows. Theoretical implications and observational consequences are briefly discussed.

Observation of the magnetic flux and three-dimensional structure of skyrmion lattices by electron holography

Abstract: Skyrmions are nanoscale spin textures that are viewed as promising candidates as information carriers in future spintronic devices. Skyrmions have been observed using neutron scattering and microscopy techniques. Real-space imaging using electrons is a straightforward way to interpret spin configurations by detecting the phase shifts due to electromagnetic fields. Here, we report the first observation by electron holography of the magnetic flux and the three-dimensional spin configuration of a skyrmion lattice in Fe(0.5)Co(0.5)Si thin samples. The magnetic flux inside and outside a skyrmion was directly visualized and the handedness of the magnetic flux flow was found to be dependent on the direction of the applied magnetic field. The electron phase shifts φ in the helical and skyrmion phases were determined using samples with a stepped thickness t (from 55 nm to 510 nm), revealing a linear relationship (φ = 0.00173 t). The phase measurements were used to estimate the three-dimensional structures of both the helical and skyrmion phases, demonstrating that electron holography is a useful tool for studying complex magnetic structures and for three-dimensional, real-space mapping of magnetic fields.

The effect of magnetic topology on thermally-driven winds: towards a general formulation of the braking law

Abstract: Stellar winds are thought to be the main process responsible for the spin down of main-sequence stars. The extraction of angular momentum by a magnetized wind has been studied for decades, leading to several formulations for the resulting torque. However, previous studies generally consider simple dipole or split monopole stellar magnetic topologies. Here we consider in addition to a dipolar stellar magnetic field, both quadrupolar and octupolar configurations, while also varying the rotation rate and the magnetic field strength. 60 simulations made with a 2.5D, cylindrical and axisymmetric set-up and computed with the PLUTO code were used to find torque formulations for each topology. We further succeed to give a unique law that fits the data for every topology by formulating the torque in terms of the amount of open magnetic flux in the wind. We also show that our formulation can be applied to even more realistic magnetic topologies, with examples of the Sun in its minimum and maximum phase as observed at the Wilcox Solar Observatory, and of a young K-star (TYC-0486- 4943-1) whose topology has been obtained by Zeeman-Doppler Imaging (ZDI).

Numerical Simulations of Active Region Scale Flux Emergence: From Spot Formation to Decay

Abstract: We present numerical simulations of active region scale flux emergence covering a time span of up to 6 days. Flux emergence is driven by a bottom boundary condition that advects a semi-torus of magnetic field with 1.7 × 1022 Mx flux into the computational domain. The simulations show that, even in the absence of twist, the magnetic flux is able the rise through the upper 15.5 Mm of the convection zone and emerge into the photosphere to form spots. We find that spot formation is sensitive to the persistence of upflows at the bottom boundary footpoints, i.e., a continuing upflow would prevent spot formation. In addition, the presence of a torus-aligned flow (such flow into the retrograde direction is expected from angular momentum conservation during the rise of flux ropes through the convection zone) leads to a significant asymmetry between the pair of spots, with the spot corresponding to the leading spot on the Sun being more axisymmetric and coherent, but also forming with a delay relative to the following spot. The spot formation phase transitions directly into a decay phase. Subsurface flows fragment the magnetic field and lead to intrusions of almost field free plasma underneath the photosphere. When such intrusions reach photospheric layers, the spot fragments. The timescale for spot decay is comparable to the longest convective timescales present in the simulation domain. We find that the dispersal of flux from a simulated spot in the first two days of the decay phase is consistent with self-similar decay by turbulent diffusion.

Asymmetric Coil Sets for Wireless Stationary EV Chargers With Large Lateral Tolerance by Dominant Field Analysis

Abstract: Asymmetric coil sets for wireless stationary electric vehicle (EV) chargers, which has significantly larger lateral tolerance than previous ones, is proposed. The pick-up coil set is much smaller than the power supply coil set, thereby allowing large lateral and longitudinal displacements as well as robustness to air-gap displacement. Electromagnetic field (EMF) is reasonably reduced by arranging magnetic poles along the EV's moving direction so that alternating magnetic flux through adjacent poles cancels each other. A dominant field analysis useful for complex vector magnetic flux simulation is newly proposed, which is applicable to any resonating coils of an inductive power transfer system (IPTS). Furthermore, a hysteresis loss model is suggested, which appropriately reflects the partial core saturation on a system analysis. A prototype IPTS including the proposed coil sets were designed and successfully verified by experiments. In the quick charging mode, maximum output power of 15 kW, large lateral displacement of 40 cm, longitudinal displacement of 20 cm, air gap of 15 cm were achieved, and low EMF of 6.1 μT at 20 kHz was achieved in the normal charging mode of 5 kW.

Formation of a double-decker magnetic flux rope in the sigmoidal solar active region 11520

Abstract: In this paper, we address the formation of a magnetic flux rope (MFR) that erupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15. Through analyzing the long-term evolution of the associated active region observed by the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, it is found that the twisted field of an MFR, indicated by a continuous S-shaped sigmoid, is built up from two groups of sheared arcades near the main polarity inversion line a half day before the eruption. The temperature within the twisted field and sheared arcades is higher than that of the ambient volume, suggesting that magnetic reconnection most likely works there. The driver behind the reconnection is attributed to shearing and converging motions at magnetic footpoints with velocities in the range of 0.1-0.6 km s(-1). The rotation of the preceding sunspot also contributes to the MFR buildup. Extrapolated three-dimensional non-linear force-free field structures further reveal the locations of the reconnection to be in a bald-patch region and in a hyperbolic flux tube. About 2 hr before the eruption, indications of a second MFR in the form of an S-shaped hot channel are seen. It lies above the original MFR that continuously exists and includes a filament. The whole structure thus makes up a stable double-decker MFR system for hours prior to the eruption. Eventually, after entering the domain of instability, the high-lying MFR impulsively erupts to generate a fast coronal mass ejection and X-class flare; while the low-lying MFR remains behind and continuously maintains the sigmoidicity of the active region.

Reduction of Low Space Harmonics for the Fractional Slot Concentrated Windings Using a Novel Stator Design

Abstract: Using magnetic flux barriers in the stator yoke of electric machines with fractional slots, tooth-concentrated winding, it is possible to reduce or even to cancel some space harmonics of low order in the air-gap flux density resulting in lower rotor losses induced by the armature reaction field. In this paper, this new technique is applied during the design and analysis of two permanent magnet machines with different 12-teeth/10-poles concentrated windings. Considering the main machine performances, such as the electromagnetic torque, machine losses, and also the field-weakening capability, the new stator design shows significant advantages over the conventional design. According to the new technique, a prototype machine is built and some measurement results are given.

Magnetic flux transport by dipolarizing flux bundles

Abstract: A dipolarizing flux bundle (DFB) is a small magnetotail flux tube (typically 65% of BBF flux transport, even though they last only ~30% as long as BBFs. The rate of DFB flux transport increases with proximity to Earth and to the premidnight sector, as well as with geomagnetic activity and distance from the neutral sheet. Under the latter two conditions, the total flux transport by a typical DFB also increases. Dipolarizing flux bundles appear more often during increased geomagnetic activity. Since BBFs have been previously shown to be the major flux transporters in the tail, we conclude that DFBs are the dominant drivers of this transport. The occurrence rate of DFBs as a function of location and geomagnetic activity informs us about processes that shape global convection and energy conversion.

Computing the reconnection rate in turbulent kinetic layers by using electron mixing to identify topology

Abstract: Three-dimensional kinetic simulations of magnetic reconnection for parameter regimes relevant to the magnetopause current layer feature the development of turbulence, driven by the magnetic and velocity shear, and dominated by coherent structures including flux ropes, current sheets, and flow vortices. Here, we propose a new approach for computing the global reconnection rate in the presence of this complexity. The mixing of electrons originating from separate sides of the magnetopause layer is used as a proxy to rapidly identify the magnetic topology and track the evolution of magnetic flux. The details of this method are illustrated for an asymmetric current layer relevant to the subsolar magnetopause and for a flow shear dominated layer relevant to the lower latitude magnetopause. While the three-dimensional reconnection rates show a number of interesting differences relative to the corresponding two-dimensional simulations, the time scale for the energy conversion remains very similar. These results suggest that the mixing of field lines between topologies is more easily influenced by kinetic turbulence than the physics responsible for the energy conversion.

Magnetic flux concentration and zonal flows in magnetorotational instability turbulence

Abstract: Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies tomore » the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.« less

Magnetic Flux Transport at the Solar Surface

Abstract: After emerging to the solar surface, the Sun’s magnetic field displays a complex and intricate evolution. The evolution of the surface field is important for several reasons. One is that the surface field, and its dynamics, sets the boundary condition for the coronal and heliospheric magnetic fields. Another is that the surface evolution gives us insight into the dynamo process. In particular, it plays an essential role in the Babcock-Leighton model of the solar dynamo. Describing this evolution is the aim of the surface flux transport model. The model starts from the emergence of magnetic bipoles. Thereafter, the model is based on the induction equation and the fact that after emergence the magnetic field is observed to evolve as if it were purely radial. The induction equation then describes how the surface flows—differential rotation, meridional circulation, granular, supergranular flows, and active region inflows—determine the evolution of the field (now taken to be purely radial). In this paper, we review the modeling of the various processes that determine the evolution of the surface field. We restrict our attention to their role in the surface flux transport model. We also discuss the success of the model and some of the results that have been obtained using this model.

Structures of Interplanetary Magnetic Flux Ropes and Comparison with Their Solar Sources

Abstract: Whether a magnetic flux rope is pre-existing or formed in situ in the Sun's atmosphere, there is little doubt that magnetic reconnection is essential to release the flux rope during its ejection. During this process, the question remains: how does magnetic reconnection change the flux-rope structure? In this work, we continue with the original study of Qiu et al. by using a larger sample of flare-coronal mass ejection (CME)-interplanetary CME (ICME) events to compare properties of ICME/magnetic cloud (MC) flux ropes measured at 1 AU and properties of associated solar progenitors including flares, filaments, and CMEs. In particular, the magnetic field-line twist distribution within interplanetary magnetic flux ropes is systematically derived and examined. Our analysis shows that, similar to what was found before, for most of these events, the amount of twisted flux per AU in MCs is comparable with the total reconnection flux on the Sun, and the sign of the MC helicity is consistent with the sign of the helicity of the solar source region judged from the geometry of post-flare loops. Remarkably, we find that about half of the 18 magnetic flux ropes, most of them associated with erupting filaments, have a nearly uniform and relatively low twist distribution from the axis to the edge, and the majority of the other flux ropes exhibit very high twist near the axis, up to 5 turns per AU, which decreases toward the edge. The flux ropes are therefore not linearly force-free. We also conduct detailed case studies showing the contrast of two events with distinct twist distribution in MCs as well as different flare and dimming characteristics in solar source regions, and discuss how reconnection geometry reflected in flare morphology may be related to the structure of the flux rope formed on the Sun.

Catastrophe versus instability for the eruption of a toroidal solar magnetic flux rope

Abstract: The onset of a solar eruption is formulated here as either a magnetic catastrophe or as an instability. Both start with the same equation of force balance governing the underlying equilibria. Using a toroidal flux rope in an external bipolar or quadrupolar field as a model for the current-carrying flux, we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for several representative evolutionary sequences in the stable domain of parameter space. We verify that this catastrophe and the torus instability occur at the same point; they are thus equivalent descriptions for the onset condition of solar eruptions.

Laser Cutting and Mechanical Cutting of Electrical Steels and its Effect on the Magnetic Properties

Abstract: It is well established that laser cutting or mechanical cutting of nonoriented electrical steel causes structural changes at the cutting edge, which finally affect the magnetic properties. During mechanical cutting, plastic deformation appears in the zone near the cutting edge. On the contrary, laser cutting induces thermal stress due to temperature gradients within the material during processing, which finally also results in deterioration of the magnetic properties. The knowledge of the type of the deterioration mechanism and the degree of the deterioration of magnetic property deterioration mechanisms is important for designing electrical machines in terms of magnetic field and loss calculations. In this paper, the effect of cutting on the magnetic flux distribution for mechanical cutting as well as solid state laser cutting is calculated and analyzed space-resolved using the data obtained from investigations by neutron grating interferometry. In addition, the resulting changes of magnetization behavior, i.e., the character of the $B$ versus $H$ curve, were studied. It will be demonstrated that the deterioration of the magnetic properties depends on the geometry of the parts at cutting. It is shown that the nature of the resulting spatial distribution of the magnetic flux is different for mechanical cutting and cutting by laser. By mechanical cutting, a drop of the magnetic flux in the region at the cutting edge appears. Through cutting by laser, the observed decrease of the magnetic flux $B$ is observed over the total width of the strip. The decrease of B at cutting of small parts by laser is remarkably different at the cutting edges, which are opposite to each other. Finally, the observed magnetic behavior is correlated to the different character of the induced residual stresses by mechanical cutting and cutting by laser.

Distribution of electric currents in solar active regions

Abstract: There has been a long-standing debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents.

One-Dimensional Helical Transport in Topological Insulator Nanowire Interferometers

Abstract: The discovery of three-dimensional (3D) topological insulators opens a gateway to generate unusual phases and particles made of the helical surface electrons, proposing new applications using unusual spin nature. Demonstration of the helical electron transport is a crucial step to both physics and device applications of topological insulators. Topological insulator nanowires, of which spin-textured surface electrons form 1D band manipulated by enclosed magnetic flux, offer a unique nanoscale platform to realize quantum transport of spin-momentum locking nature. Here, we report an observation of a topologically protected 1D mode of surface electrons in topological insulator nanowires existing at only two values of half magnetic quantum flux (±h/2e) due to a spin Berry’s phase (π). The helical 1D mode is robust against disorder but fragile against a perpendicular magnetic field breaking-time-reversal symmetry. This result demonstrates a device with robust and easily accessible 1D helical electronic states f...

Classical T Tauri stars : magnetic fields, coronae, and star-disc interactions

Abstract: The magnetic fields of young stars set their coronal properties and control their spin evolution via the star–disc interaction and outflows. Using 14 magnetic maps of 10 classical T Tauri stars (CTTSs) we investigate their closed X-ray emitting coronae, their open wind-bearing magnetic fields and the geometry of magnetospheric accretion flows. The magnetic fields of all the CTTSs are multipolar. Stars with simpler (more dipolar) large-scale magnetic fields have stronger fields, are slower rotators and have larger X-ray emitting coronae compared to stars with more complex large-scale magnetic fields. The field complexity controls the distribution of open and closed field regions across the stellar surface, and strongly influences the location and shapes of accretion hot spots. However, the higher order field components are of secondary importance in determining the total unsigned open magnetic flux, which depends mainly on the strength of the dipole component and the stellar surface area. Likewise, the dipole component alone provides an adequate approximation of the disc truncation radius. For some stars, the pressure of the hot coronal plasma dominates the stellar magnetic pressure and forces open the closed field inside the disc truncation radius. This is significant as accretion models generally assume that the magnetic field has a closed geometry out to the inner disc edge.

Core Loss Analysis and Calculation of Stator Permanent-Magnet Machine Considering DC-Biased Magnetic Induction

Abstract: In this paper, core loss characteristics of stator permanent-magnet (PM) machines are analyzed and calculated, in which the unique dc-biased magnetic induction is taken into account. The flux density variation patterns in some key points in a doubly salient PM (DSPM) machine are analyzed by the two-dimensional time-stepping finite-element method. To calculate the core loss of stator and other minor hysteresis loops under dc magnetic bias, core loss characteristics of silicon steel sheet under dc-biased induction condition are first tested by a simple test system based on traditional Epstein frame and a functional relationship between the change of dc-biased induction and hysteresis loss is proposed. Two core loss calculation methods considering rotational magnetization based on flux density waveform are improved to take the influence of dc-biased magnetic induction into consideration. The calculation results are verified by experiments on a prototype DSPM machine. The conclusions are also applicable to other kinds of stator PM machines.

Convection causes enhanced magnetic turbulence in accretion disks in outburst

Abstract: We present the results of local, vertically stratified, radiation magnetohydrodynamic (MHD) shearing box simulations of magneto-rotational instability (MRI) turbulence appropriate for the hydrogen ionizing regime of dwarf nova and soft X-ray transient outbursts. We incorporate the frequency-integrated opacities and equation of state for this regime, but neglect non-ideal MHD effects and surface irradiation, and do not impose net vertical magnetic flux. We find two stable thermal equilibrium tracks in the effective temperature versus surface mass density plane, in qualitative agreement with the S-curve picture of the standard disk instability model. We find that the large opacity at temperatures near 104?K, a corollary of the hydrogen ionization transition, triggers strong, intermittent thermal convection on the upper stable branch. This convection strengthens the magnetic turbulent dynamo and greatly enhances the time-averaged value of the stress to thermal pressure ratio ?, possibly by generating vertical magnetic field that may seed the axisymmetric MRI, and by increasing cooling so that the pressure does not rise in proportion to the turbulent dissipation. These enhanced stress to pressure ratios may alleviate the order of magnitude discrepancy between the ?-values observationally inferred in the outburst state and those that have been measured from previous local numerical simulations of magnetorotational turbulence that lack net vertical magnetic flux.

Numerical simulations of active region scale flux emergence: From spot formation to decay

Abstract: We present numerical simulations of active region scale flux emergence covering a time span of up to 6 days. Flux emergence is driven by a bottom boundary condition that advects a semi-torus of magnetic field with $1.7 \times 10^{22}$ Mx flux into the computational domain. The simulations show that, even in the absense of twist, the magnetic flux is able the rise through the upper $15.5$ Mm of the convection zone and emerge into the photosphere to form spots. We find that spot formation is sensitive to the persistence of upflows at the bottom boundary footpoints, i.e. a continuing upflow would prevent spot formation. In addition, the presence of a torus-aligned flow (such flow into the retrograde direction is expected from angular momentum conservation during the rise of flux ropes through the convection zone) leads to a significant asymmetry between the pair of spots, with the spot corresponding to the leading spot on the Sun being more axisymmetric and coherent, but also forming with a delay relative to the following spot. The spot formation phase transitions directly into a decay phase. Subsurface flows fragment the magnetic field and lead to intrusions of almost field free plasma underneath the photosphere. When such intrusions reach photospheric layers, the spot fragments. The time scale for spot decay is comparable to the longest convective time scales present in the simulation domain. We find that the dispersal of flux from a simulated spot in the first two days of the decay phase is consistent with self-similar decay by turbulent diffusion.

The Most Magnetic Stars

Abstract: Observations of magnetic A, B and O stars show that the poloidal mag- netic flux per unit massp/M appears to have an upper bound of approximately 10 6.5 Gcm 2 g 1 . A similar upper bound to the total flux per unit mass is found for the magnetic white dwarfs even though the highest magnetic field strengths at their surfaces are much larger. For magnetic A and B stars there also appears to be a well defined lower bound below which the incidence of magnetism declines rapidly. Accord- ing to recent hypotheses, both groups of stars may result from merging stars and owe their strong magnetism to fields generated by a dynamo mechanism as they merge. We postulate a simple dynamo that generates magnetic field from differential rotation. We limit the growth of magnetic fields by the requirement that the poloidal field stabilizes the toroidal and vice versa. While magnetic torques dissipate the differential rotation, toroidal field is generated from poloidal by an dynamo. We further suppose that mechanisms that lead to the decay of toroidal field lead to the generation of poloidal. Both poloidal and toroidal fields reach a stable configuration which is independent of the size of small initial seed fields but proportional to the initial differential rotation. We pose the hypothesis that strongly magnetic stars form from the merging of two stellar objects. The highest fields are generated when the merge introduces differential rotation that amounts to critical break up velocity within the condensed object. Cali- bration of a simplistic dynamo model with the observed maximum flux per unit mass for main-sequence stars and white dwarfs indicates that about 1.5×10 4 of the decay- ing toroidal flux must appear as poloidal. The highest fields in single white dwarfs are generated when two degenerate cores merge inside a common envelope or when two white dwarfs merge by gravitational-radiation angular momentum loss. Magnetars are the most magnetic neutron stars. Though these are expected to form directly from single stars, their magnetic flux to mass ratio indicates that a similar dynamo, driven by differential rotation acquired at their birth, may also be the source of their strong magnetism.

Ultra-high quality factors in superconducting niobium cavities in ambient magnetic fields up to 190 mG

Abstract: Ambient magnetic field, if trapped in the penetration depth, leads to the residual resistance and therefore sets the limit for the achievable quality factors in superconducting niobium resonators for particle accelerators. Here, we show that a complete expulsion of the magnetic flux can be performed and leads to: (1) record quality factors Q > 2 × 1011 up to accelerating gradient of 22 MV/m; (2) Q ∼ 3 × 1010 at 2 K and 16 MV/m in up to 190 mG magnetic fields. This is achieved by large thermal gradients at the normal/superconducting phase front during the cooldown. Our findings open up a way to ultra-high quality factors at low temperatures and show an alternative to the sophisticated magnetic shielding implemented in modern superconducting accelerators.

The Solar Internetwork. I. Contribution to the Network Magnetic Flux

Abstract: The magnetic network (NE) observed on the solar surface harbors a sizable fraction of the total quiet Sun flux. However, its origin and maintenance are not well known. Here we investigate the contribution of internetwork (IN) magnetic fields to the NE flux. IN fields permeate the interior of supergranular cells and show large emergence rates. We use long-duration sequences of magnetograms acquired by Hinode and an automatic feature tracking algorithm to follow the evolution of NE and IN flux elements. We find that 14% of the quiet Sun (QS) flux is in the form of IN fields with little temporal variations. IN elements interact with NE patches and modify the flux budget of the NE either by adding flux (through merging processes) or by removing it (through cancellation events). Mergings appear to be dominant, so the net flux contribution of the IN is positive. The observed rate of flux transfer to the NE is 1.5 × 1024 Mx day–1 over the entire solar surface. Thus, the IN supplies as much flux as is present in the NE in only 9-13 hr. Taking into account that not all the transferred flux is incorporated into the NE, we find that the IN would be able to replace the entire NE flux in approximately 18-24 hr. This renders the IN the most important contributor to the NE, challenging the view that ephemeral regions are the main source of flux in the QS. About 40% of the total IN flux eventually ends up in the NE.

On Polar Magnetic Field Reversal and Surface Flux Transport During Solar Cycle 24

Abstract: As each solar cycle progresses, remnant magnetic flux from active regions (ARs) migrates poleward to cancel the old-cycle polar field. We describe this polarity reversal process during Cycle 24 using four years (2010.33--2014.33) of line-of-sight magnetic field measurements from the Helioseismic and Magnetic Imager. The total flux associated with ARs reached maximum in the north in 2011, more than two years earlier than the south; the maximum is significantly weaker than Cycle 23. The process of polar field reversal is relatively slow, north-south asymmetric, and episodic. We estimate that the global axial dipole changed sign in October 2013; the northern and southern polar fields (mean above 60$^\circ$ latitude) reversed in November 2012 and March 2014, respectively, about 16 months apart. Notably, the poleward surges of flux in each hemisphere alternated in polarity, giving rise to multiple reversals in the north. We show that the surges of the trailing sunspot polarity tend to correspond to normal mean AR tilt, higher total AR flux, or slower mid-latitude near-surface meridional flow, while exceptions occur during low magnetic activity. In particular, the AR flux and the mid-latitude poleward flow speed exhibit a clear anti-correlation. We discuss how these features can be explained in a surface flux transport process that includes a field-dependent converging flow toward the ARs, a characteristic that may contribute to solar cycle variability.

Simulating the in Situ Condensation Process of Solar Prominences

Abstract: Prominences in the solar corona are a hundredfold cooler and denser than their surroundings, with a total mass of 1013 up to 1015 g. Here, we report on the first comprehensive simulations of three-dimensional, thermally and gravitationally stratified magnetic flux ropes where in situ condensation to a prominence occurs due to radiative losses. After a gradual thermodynamic adjustment, we witness a phase where runaway cooling occurs while counter-streaming shearing flows drain off mass along helical field lines. After this drainage, a prominence-like condensation resides in concave upward field regions, and this prominence retains its overall characteristics for more than two hours. While condensing, the prominence establishes a prominence-corona transition region where magnetic field-aligned thermal conduction is operative during the runaway cooling. The prominence structure represents a force-balanced state in a helical flux rope. The simulated condensation demonstrates a right-bearing barb, as a remnant of the drainage. Synthetic images at extreme ultraviolet wavelengths follow the onset of the condensation, and confirm the appearance of horns and a three-part structure for the stable prominence state, as often seen in erupting prominences. This naturally explains recent Solar Dynamics Observatory views with the Atmospheric Imaging Assembly on prominences in coronal cavities demonstrating horns.

Magnetic Flux Transport at the Solar Surface

Abstract: After emerging to the solar surface, the Sun's magnetic field displays a complex and intricate evolution. The evolution of the surface field is important for several reasons. One is that the surface field, and its dynamics, sets the boundary condition for the coronal and heliospheric magnetic fields. Another is that the surface evolution gives us insight into the dynamo process. In particular, it plays an essential role in the Babcock-Leighton model of the solar dynamo. Describing this evolution is the aim of the surface flux transport model. The model starts from the emergence of magnetic bipoles. Thereafter, the model is based on the induction equation and the fact that after emergence the magnetic field is observed to evolve as if it were purely radial. The induction equation then describes how the surface flows -- differential rotation, meridional circulation, granular, supergranular flows, and active region inflows -- determine the evolution of the field (now taken to be purely radial). In this paper, we review the modeling of the various processes that determine the evolution of the surface field. We restrict our attention to their role in the surface flux transport model. We also discuss the success of the model and some of the results that have been obtained using this model.

Slow Rise and Partial Eruption of a Double-decker Filament. II. A Double Flux Rope Model

Abstract: Force-free equilibria containing two vertically arranged magnetic flux ropes of like chirality and current direction are considered as a model for split filaments/prominences and filament-sigmoid systems. Such equilibria are constructed analytically through an extension of the methods developed in Titov & Demoulin and numerically through an evolutionary sequence including shear flows, flux emergence, and flux cancellation in the photospheric boundary. It is demonstrated that the analytical equilibria are stable if an external toroidal (shear) field component exceeding a threshold value is included. If this component decreases sufficiently, then both flux ropes turn unstable for conditions typical of solar active regions, with the lower rope typically becoming unstable first. Either both flux ropes erupt upward, or only the upper rope erupts while the lower rope reconnects with the ambient flux low in the corona and is destroyed. However, for shear field strengths staying somewhat above the threshold value, the configuration also admits evolutions which lead to partial eruptions with only the upper flux rope becoming unstable and the lower one remaining in place. This can be triggered by a transfer of flux and current from the lower to the upper rope, as suggested by the observations of a split filament in Paper I. It can also result from tether-cutting reconnection with the ambient flux at the X-type structure between the flux ropes, which similarly influences their stability properties in opposite ways. This is demonstrated for the numerically constructed equilibrium.

Flux Balancing of Isolation Transformers and Application of “The Magnetic Ear” for Closed-Loop Volt–Second Compensation

Abstract: Semiconductor switches possess nonideal behavior which, in case of isolated dc-dc converters, can generate dc-voltage components which are then applied to the isolation transformer. This dc-voltage component is translated into a dc flux density component in the transformer core, increasing the risk of driving the core into saturation. In this paper, a novel noninvasive flux density measurement principle, called “The Magnetic Ear,” based on sharing of magnetic path between the main and an auxiliary core is proposed. The active compensation of the transformer's dc magnetization level using this transducer is experimentally verified. Additionally, a classification of the previously reported magnetic flux measurement and balancing concepts is performed.